As semiconductor device dimensions in an integrated circuit (IC) become smaller, a specification associated with how the IC should be fabricated becomes more restrictive with respect to the amount of error that is allowable in the process. The specification may provide the requirements that a photomask, also known as a mask or reticle, must meet in order to be used to fabricate semiconductor devices on a wafer. For example, a specification may include the requirements for pattern position accuracy, feature size control and defect density for a specific manufacturing process.
The pattern position accuracy and feature size control on an individual photomask may be measured by a metrology tool. Typically, a metrology tool should have no more than ten percent error relative to the specification in order to provide quality assurance. Today, it is common for advanced photomasks to have specifications of less than forty nanometers, which may limit the requirements for tool-to-tool matching to less than five nanometers. A metrology tool, therefore, must be able to accurately measure a feature on a photomask in order to meet the tool requirements in the specification.
The accuracy of a metrology tool, however, may be affected by systematic and/or random error factors. Systematic factors, such as X-mag, Y-mag and orthogonality, may be used to match different metrology tools to each other and to customer specific grids or footprints. A transfer standard artifact may be the physical vehicle used for tool matching and grid definition to ensure consistent results between different tools in the same manufacturing facility or in different manufacturing facilities. Furthermore, the metrology tools must be kept within process control standards by maintaining.
Tool matching, grid transfers and statistical process control methods may also be effected by random errors that limit the ability to accurately and directly apply measured data from the metrology tool to a higher level of certainty. Therefore, as specifications for a photomask become more challenging, the ability to reduce and resolve random error sources is critical to achieving better results from the metrology tools. If the random errors in a metrology tool can be reduced, the metrology tool may provide more repeatable and accurate measurements for the features on a photomask.